There is extreme inter-individual variation in human platelet reactivity, which likely contributes to occlusion of coronary and cerebral arteries upon atherosclerotic plaque rupture in some individuals, whereas other individuals repair the wound without occluding the vessel. However, there is a lack of understanding of the molecular and genetic mechanisms responsible for this variation in platelet function. Our previous genome wide scan and platelet RNA expression studies have elucidated mechanisms for epinephrine-induced platelet aggregation, identified mRNA expression differences, as well as unexpected evidence that microRNAs (miRNAs) regulate expression of the platelet secretory protein, VAMP8. Additional analyses suggest that alternate exon splicing and genomic copy number variation also contribute to variation in platelet gene expression. Although our platelet RNA profiling study included 29 subjects, it was limited in its ability to identify genes involved in the platelet response to agonists other than epinephrine. We now propose to recall 180 subjects for new platelet phenotyping, preparation of leukocyte-depleted platelet RNA and microarray profiling of mRNA and miRNA. Rigorous bioinformatic and statistical approaches will minimize false positives, and candidate mRNAs and miRNAs will be characterized experimentally. This approach will allow definition of genes regulating the platelet response to specific agonists (ADP, CRP, collagen, TRAP, U46619, ristocetin, CD9 and epinephrine), as well as genes common to multiple platelet stimuli, and permit assessment of the effects of gender and ethnicity (Aim 1). In Aims 2-4 we will characterize molecular mechanisms regulating mRNA levels in platelets of differing reactivity. Our analyses will identify associations between platelet reactivity and alternately spliced exons, thus serving as a screen for functional domains in the platelet protein (Aim 2). We will determine platelet miRNA profiles and characterize the function of differentially expressed miRNAs in platelets of differing reactivity (Aim 3). By correlating RNA expression data with our prior GWAS genotyping on these same subjects, we will address the role of copy number variation on platelet gene expression (Aim 4). We will also generate public web tools that provide the relative expression of platelet mRNAs and miRNAs according to gender, race and age. This research will result in new insights into platelet physiology, enhance our understanding of the genetics of platelet gene expression, facilitate the selection of gender and race-specific biomarkers for thrombosis risk, and provide useful tools for other platelet researchers.